1. Field of the Invention
The present invention relates to a pattern measuring apparatus and a pattern measuring method which use an electron beam. In particular, the present invention relates to a pattern measuring apparatus and a pattern measuring method which are capable of measuring the distance between pattern edges with better reproducibility.
2. Description of the Prior Art
A pattern measuring method using a scanning electron microscope is employed as a method of measuring a line width of a pattern. The scanning electron microscope is configured: to radiate incident electrons while scanning the electron scanning range; to acquire secondary electrons, which are emitted from a sample, by use of a scintillator; to acquire image data by converting the amount of acquired electrons to brightness; and to display the image on a display unit.
In order to control characteristics of a semiconductor device by using such a scanning electron microscope, a generally employed practice is to check pattern formation to determine whether a line width of each line pattern and a distance between two neighboring patterns fall within ranges of design standard values. Pattern line widths and distances between patterns are controlled by use of the following procedure. To begin with, an XY stage is moved to position the scanning electron microscope on a predetermine position on patterns formed on a photomask. Thereafter, an electron beam is radiated on the patterns in a specified range which covers a position of measurement. Thus, an image representing the distribution of brightness is acquired on the basis of secondary electrons reflected from the irradiated points. Subsequently, a waveform representing the distribution of brightness is acquired from the brightness information on the measurement points. By analyzing the acquired waveform, the positions of edges of each pattern are found to determine the line width of the pattern. In addition, the distance between two edges of respective two patterns is calculated from values in the acquired image data. Subsequently, it is judged whether or not each of the line width of each pattern and the distance between the patterns fall within a range of a tolerance. The result of the judgment is used as criteria for determining whether the quality of the photomask is satisfactory or not, and as information on a process feedback for the preceding processes.
The measurement of pattern line widths and distances between patterns as described above is important for the photomask manufacturing process. Thus, various methods of measuring a line width and the like have been proposed.
In general, a position at which the brightness corresponding to the amount of secondary electrons reaches a maximum gradient is defined as the position of an edge of the pattern. In contrast, Japanese Unexamined Patent Application Publication No. Hei. 5-296754 discloses an edge detecting method in which a position at which a secondary electron signal shows a minimum value is defined as the position of an edge.
As described above, the method in which a position at which the brightness is at the maximum gradient is defined as the position of an edge, and the method in which a position at which the secondary electron signal is at the minimum value is defined as the position of an edge are employed to measure the line width of a pattern by use of a scanning electron microscope.
In such pattern edge detection, a range for edge measurement is set wider, and the position of an edge is determined by use of multiple detection values. This enables the position of the edge to be detected with better reproducibility even if a region specified as a measurement target is somewhat off the exact spot.
However, it is difficult to stably detect the position of an edge in a narrower portion of a pattern, for instance, an extreme end portion of a line pattern, because a wide measurement range cannot be used. In addition, when a region specified as a measurement target is not exactly on a desired spot, the region may or not may include a curved portion in a corner section of the pattern. This makes it difficult to detect the position of the edge with high reproducibility.